how old?

I've been reading through the different threads about feeders. One of the Stickis says that rats shouldn't be bred until they are about 4-5 months old. So if I look into getting rats from a pet store, what size would be ready? Small, Medium, Large? Are the Large Rats going to produce smaller litters? Are they going to produce a litter at all when they get that old?

If a rat is too old, they won't be able to reproduce. I'd go with smalls or mediums.

That's what I figured, THANKS!!!!